1. Technical Field
The present invention relates to the use of a gas bubble system to remove fouling materials from the surface of membranes used in filtration systems and the like.
2. Background Art
A variety of membrane filtration systems are known and many of these use pressurised systems operating at high transmembrane pressures (TMP) to produce effective filtering and high filtrate flux. These systems are highly effective but are also expensive to produce, operate and maintain. Simpler systems using membrane arrays freely mounted vertically in a tank and using suction applied to the fibre lumens to produce TMP have also been developed, however, these systems have been found in the past to be less effective than the pressurised systems.
Examples of such known systems are illustrated in U.S. Pat. No. 5,192,456 to Ishida et al, U.S. Pat. No. 5,248,424 to Cote et al and WO 97/06880 to Zenon Environmental Inc.
The Ishida et al patent describes an activated sludge treating apparatus where air flow is used to clean the outer surface of the filter membrane. In this arrangement the air blower used for biological treatment of the waste water is also used as a secondary agitation source to clean the surface of the membranes. The membrane modules are of the plate type. The membranes also have a low packing density and thus do not have the problems associated with cleaning tightly packed fibre bundles. Air is bubbled from beneath the modules and is supplied externally from the membrane array.
The Cote et al patent again describes a system of cleaning arrays of fibres. In this case the fibres are mounted in a skein to form an inverted U-shaped or parabolic array and the air is introduced below the array to produce bubbles which contact the fibres with such force they keep the surfaces relatively free of attached microorganisms and deposits of inanimate particles. The fibres are freely swayable as they are only attached at either end and this assists removal of deposits on their outer surface. The bubbles of gas/air flow are provided from a source external of the fibre bundle and move generally transverse to the lengths of fibre. This limits the depth of fibre bundle which can be effectively cleaned.
The invention disclosed in the Zenon Environmental, Inc. PCT Application No. WO 97/06880 is closely related to the Cote et al patent. In this document the fibres are unconfined, vertically arranged and dimensioned to be slightly longer than the distance between the opposed faces of the headers into which the fibre ends are mounted to allow for swaying and independent movement of the individual fibres. The skein is aerated with a gas distribution means which produces a mass of bubbles which serve to scrub the outer surface of the vertically arranged fibres as they rise upwardly through the skein.
Our own International Patent Application WO96/07470 describes an earlier method of cleaning membranes using a gas backwash to dislodge material from the membrane walls by applying a gas pressure to the filtrate side of the membranes and then rapidly decompressing the shell surrounding the feed side of the membranes. Feed is supplied to the shell while this gas backwash is taking place to cause turbulence and frothing around the membrane walls resulting in further dislodgment of accumulated solids.
The present invention relates particularly to a plurality of porous membranes arranged to form a membrane module arranged in a relatively tightly packed bundle. These porous membranes may be in the form of fibres or plate type membranes as described in the above prior art.
The present invention seeks to overcome or at least ameliorate the problems of the prior art by providing a simple effective system and method for removing fouling materials from the surface of the porous membranes by use of gas bubbles.
According to one aspect, the present invention provides a method of removing fouling materials from the surface of a plurality of porous membranes arranged in a membrane module, the porous membranes forming an array, the module having a header used to mount the membranes, the header connected to a source of pressurized gas, the method comprising providing, through the header gas bubbles in a uniform distribution relative to the porous membrane array such that said bubbles move past the surfaces of said membranes to dislodge fouling materials therefrom, said membranes being arranged in close proximity to one another and mounted to prevent excessive movement therebetween. The porous membranes may comprise hollow fibre membranes. Preferably, the fibre membranes are arranged in bundles surrounded by a perforated cage which serves to prevent said excessive movement therebetween.
According to a second aspect, the present invention provides a membrane module comprising a plurality of porous membranes, said membranes being arranged in close proximity to one another and mounted to prevent excessive movement therebetween, the membranes forming an array, the module having a header used to mount the membranes, the header connected to a source of pressurized gas so as permit formation of gas bubbles such that, in use, said gas moves through said header, and said bubbles move past the surfaces of said membranes to dislodge fouling materials therefrom.
The gas bubbles may be provided from within the module by a variety of methods including gas distribution holes or openings in the header, a porous tube located within the module or a tube or tubes positioned to output gas within the module, the tubes may be in the form of a comb of tubes containing holes which sit within the module. Another method of providing gas bubbles includes creating gas in-situ by means of spark type ozone generators or the like. Further types of gas provision are detailed below and in the preferred embodiments of the invention.
According to one preferred form, the present invention provides a method of removing fouling materials from the surface of a plurality of porous hollow fibre membranes mounted and extending longitudinally in an array to form a membrane module, said membranes being arranged in close proximity to one another and mounted to prevent excessive movement therebetween, the method comprising the steps of providing, from within said array, via the header connected to a source of pressurized gas, uniformly distributed gas bubbles, said distribution being such that said bubbles pass substantially uniformly between each membrane in said array to scour the surface of said membranes and remove accumulated solids from within the membrane module.
For preference, said membranes are mounted vertically to form said array and said bubbles pass generally parallel to the longitudinal extent of said fibres. Preferably, said uniformly distributed gas bubbles are provided at the lower end of the array. Optionally, a backwash may be used in conjunction with the removal process to assist solids removal from the membrane pores and outer surface of the membranes.
For preference, the membranes comprise porous hollow fibres, the fibres being fixed at each end in a header, the lower header having a plurality of holes formed therein through which gas is introduced to provide the gas bubbles. The fibres are normally sealed at the lower end and open at their upper end to allow removal of filtrate. Some of the fibres may also be used to provide bubbles of scouring gas to the array by feeding gas through selected ones of the fibres in the array. The fibres are preferably arranged in cylindrical arrays or bundles.
Filtrate is normally withdrawn from the fibres by application of suction applied thereto, however, it will be appreciated that any suitable means of providing TMP may be used. A porous sheet may be used in conjunction with the holes or separately to provide a more uniform distribution of gas bubbles. The porous sheet also provides the added advantage of preventing solids ingressing into the air supply plenum chamber.
According to a further preferred aspect, the present invention provides a membrane module comprising a plurality of porous hollow membrane fibres extending longitudinally between and mounted at each end to a respective potting head, said membrane fibres being arranged in close proximity to one another and mounted to prevent excessive movement therebetween, one of said potting heads having a uniform distributed array of aeration holes formed therein and said fibres being substantially uniformly mounted in said one potting head relative to said aeration holes.
According to a preferred further aspect, the present invention provides a filtration system including a membrane module according to said second aspect wherein said filter module is positioned vertically in a tank containing feed liquid to be filtered, means to apply a transmembrane pressure to said fibres in said array to cause filtrate to pass through pores in said fibres and means to supply continually or intermittently a supply of pressurized gas to said aeration holes so as to produce gas bubbles which move upwardly and uniformly between said fibres to scour the outer surfaces thereof.
Optionally, when the module is contained in a separate vessel, periodic draindown of the vessel is carried out after the scouring step to remove solids accumulated during the scouring process. Apart from draindown, other methods can be used for accumulated solids removal. These include continual bleed off of concentrated feed during the filtration cycle or overflow at the top of the tank by pumping feed into the base of the tank at regular intervals at a rate sufficient to cause overflow and removal of accumulated solids. This would be typically done at the end of a backwash cycle.
It should be understood that the term xe2x80x9cgasxe2x80x9d used herein includes any gas, including air and mixtures of gases as well as ozone and the like.
It will be appreciated that the above described invention may be readily applied to our own modular microporous filter cartridges as used in our continuous microfiltration systems and described in our earlier United States Patent specification No. 5,405,528. These cartridges may be modified by providing gas distribution holes in the lower plug and providing a manifold for supplying gas to said holes such that, in use, the gas passes through the holes and forms scouring bubbles which pass upward through the filter medium. In a preferred arrangement, the filter medium would be sealed at the lower end and filtrate withdrawn under a vacuum from the upper end while the cartridge or cartridges were positioned in a tank containing the feed.
The embodiments of the invention will be described in relation to microporous fibre membranes, however, it will be appreciated that the invention is equally applicable to any form of membrane module.